1. Field of the Invention
This invention relates to compositions and the methods for using them for removing snow or ice from concrete, reinforcing concrete, and ferrous or aluminum metal surfaces without producing corrosion of the metal or the breakup of the concrete from corroding metals in the concrete.
2. History of the Prior Art
Common salt, sodium chloride, is the major chemical used for deicing highways, bridge decks, overpasses and other paved surfaces. Calcium chloride is used for this purpose to a lesser extent. Sodium chloride has the advantage that it is less expensive. Calcium chloride costs several times as much as sodium chloride, but it can melt ice or snow at much lower temperatures and has an exothermic heat of solution. Since most snow storms that affect vehicular traffic occur at about 32 degrees F., and sodium chloride is effective at a substantially lower temperatures, the lower eutectic temperature and the exothermic heat of solution of calcium chloride is not as important as the lower cost of the salt.
Both salt and calcium chloride, because of their chloride content, have deleterious affects on any steel which is exposed to these chloride solutions. The contained chlorides also create the possibility of producing chloride toxicity in any plant life which is exposed to the solutions produced by the action of chloride type deicers.
When salt or calcium chloride is applied to reinforced concrete structures, severe corrosion of the steel is produced and as the reinforcing steel corrodes, the corrosion products created put a stress on the surrounding concrete to produce pot holes in the concrete. Thus, there is not only damage to the reinforcing steel but also to the concrete. Repairing this damage costs millions of dollars for some of the major highway structures. The corrosion produced on highway vehicles and on other steel structures, which are exposed to these chloride solutions, is recognized as significant.
As an example of the toxic affect of chloride salts on plant life, in the "Biological Abstracts" of Dec. 1, 1971, No. 130264 there is an abstract of an article by Aoki et al. giving the concentration of chlorides in the growing media which produces a toxic effect on some varieties of crytomeria japonica. To quote, "Some varieties grew poorly at Cl concentration of 0.06%. At 0.14% the dry weight increased but the plant wilted due to an excessive accumulation of NaCl toward the end of their growths."
Since salt and calcium chloride are so corrosive to metal, it is necessary to use other deicers for removing snow or ice from metal surfaces. As an example, propylene glycol solutions are used to flush the snow from airplanes just prior to take off. Even though propylene glycol is a comparatively expensive chemical, it has the advantage that it is not inherently corrosive to metal and any corrosion which might occur is eliminated by the addition of corrosion inhibitors.
In order to keep railway switches working freely, it is necessary to use deicers. These deicers can not be corrosive to steel.
With such a need for an inexpensive, noncorrosive, deicing composition, it is understandable that considerable expense is involved in looking for a chemical which is relatively inexpensive, noncorrosive to ferrous metals and aluminum, has a low order of toxicity to plants and does not damage concrete. In a publication titled, "Alternative Highway Deicing Chemicals" by Dr. Stanley A. Dunn and Dr. Roy H. Schenk of the Bjorksten Research Laboratories, Inc., and based on Federal Highway Administration Report No. FHWA-RD-78-67, a summary is made of the potential of deicing chemicals which might be used to replace salt or calcium chloride as noncorrosive deicing chemicals along with the advantages and disadvantages each chemical might have. Following is a list of the chemicals suggested: sodium bicarbonate-sodium carbonate, sodium monohydrogen phosphate-sodium dihydrogen phosphate, potassium bicarbonate-potassium carbonate, potassium monohydrogen phosphate-potassium dihydrogen phosphate, tetrapotassium pyrophosphate, ammonium monohydrogen phosphate-ammonium dihydrogen phosphate, ammonium bicarbonate-ammonium carbonate, various organic compounds, such as, alcohols (methanol, ethanol, isopropanol), carboxylic acids, di-carboxylic acids, amides (urea or formamide), ketones (acetone), aldehydes, amines, ammonium carbonate, dimethyl sulfoxide, and metal organic salts (calcium and magnesium acetate). In this list, the article suggests that the most promising deicing candidates are methanol and calcium magnesium acetate (abbreviated as CMA).
Research has been performed and reported on the effect that chlorides have on the corrosion of steel and of reinforcing steel in concrete. Among these publications are the following: "Corrosion of Reinforcing Bars in Concrete" by Mozer, Bianchini and Kesler in the August 1965 issue of Journal of American Concrete Institute; "Corrosion of Reinforcing Steel" by Tremper; ASTM Special Technical Publication No. 169-A, "Significance of Tests and Properties of Concrete and Concrete Making Materials", "Concrete Variables and Corrosion Testing" by Spellman and Stratful, State of California, Department of Public Works, Division of Highways, Materials and Research Department, Research Report No. M & R 635116-6, FHWA D-3-11; "A Rapid Method of Studying Corrosion Inhibition of Steel in Concrete" by Gouda and Monfore, Journal of PCA Research and Development Laboratories, Ser. 1175, September, 1965.
A review of these publications appears to indicate the following: Concrete, itself, acts as an inhibitor to corrosion of the contained reinforcing steel. The corrosion of steel in concrete may be chemical but more commonly it is electrochemical in nature. The area where metal ions go into solution becomes the anodic region. If the metal is iron, the iron, as it goes into solution, forms ferrous ions, Fe++ plus two electrons, 2 e-. To maintain an equilibrium of electrical charges, an equivalent quantity of hydrogen is plated out at adjacent surfaces of the metal, the cathode. This thin film of hydrogen inhibits further corrosion unless the hydrogen film is removed in some manner. The anodic and cathodic reactions are represented as follows: EQU anodic Fe.fwdarw.Fe+++2e- (1) EQU cathodic 2H++2e-.fwdarw.H2 (2)
The reaction taking place at the cathode area is slow in alkaline media because the concentration of hydrogen is very low but is accelerated by the depolarizing action of dissolved oxygen. EQU 2H++1/2O2+2e.fwdarw.H2) (3) EQU Fe+H2O+1/2O2.fwdarw.Fe(OH)2 (4)
The corrosion rate is proportional to the oxygen concentration. The quantity of electricity flowing through the local cells is equivalent to the amount of metallic corrosion. With increasing anodic polarization, the overall corrosion of metal diminishes. In ordinary conditions of reinforcing steel in concrete, where pH is high and the hydrogen concentration is low and where the oxygen supply is virtually non-existent, an anodic coating builds up on the steel to stop the corrosion. When chloride deicing salts are used the protective, anodic, iron oxide and hydrogen films are removed by forming the soluble chloride compounds and thus leaving the iron or steel open to further electrochemical attack.
Gouda and Monfore made the following statement, "Since areas that corrode are anodic, valuable information may be obtained on a macroscopic scale by forcing the whole metal to be anodic. This can be accomplished by applying an external voltage between the metal as an anode and an auxilary electrode as cathode. Polarization current densities of from 1 to 1000 micro-amperes per sq. cm. are usually applied in such tests, presumably approximating the values encountered in actual local cells." The tests used in evaluating the corrosive action of various chemicals in the present application are based upon a modified version of the foregoing procedure used by Gouda and Monfore.
Metal which is above hydrogen in the emf series, such as iron or aluminum, may corrode quite readily when not imbedded in concrete, as it may be exposed to either oxygen or to hydrogen ions; although, aluminum forms an aluminum oxide coating which resists oxygen corrosion but is subject to hydrogen ion corrosion. Of course water, in the open atmosphere, will corrode iron or steel. Aluminum is also subject to corrosion from strong hydroxide solutions.
Urea has been used as a deicing compound. It is not considered to be as corrosive to ferrous metal as the chloride deicing salts and it is not as toxic to plants. Urea has the disadvantage of having a relatively high eutectic temperature in a water solution, that is, 11.3 degrees F. at a concentration in water of 32.5 percent. It also takes about 2.2 times as much urea to produce a urea solution in water, 5.5 percent, that has the same freezing point as a 2.5 percent solution of sodium chloride in water.
The eutectic temperature of common salt, sodium chloride, is -6 degrees F. It is recognized in the art of removing snow or ice from highways that a temperature difference of about 20 degrees F. between the eutectic temperature of a deicing composition in water and the air temperature is needed to melt the snow or ice. For example, the salt in water eutectic temperature of -6 degrees F. plus the 20 degree difference is equal to an air temperature of 14 degrees F. To quote from the "Snow Fighter's Handbook" from the Utah State Department of Transportation, "Remember--watch your thermometer and treat the snow accumlation accordingly. When it is 15 degrees F. or lower, forget the salting. It is too cold for it to have any noticeable effect. Use sand or other abrasive . . . "
The eutectic temperature of urea, ammonium carbonate and anmonium bicarbonate are 11.3, 5.7, and 14.9 degrees F., respectively. By adding 20 degrees to these temperatures, the practical deicing temperatures of these compounds is 31.3, 25.7 and 34.9 degrees F., respectively, and only the ammonium carbonate appears to have much value when the air temperature is much below 32 degrees.
In deicing highways, the "Snow Fighter's Handbook" for the State of Utah suggests that the deicing salt should be applied to the highway "When the ground is barely white and is wet enough to hold the salt on the roadbed." For Type 1 service, the handbook recommends applying the "straight salt at the rate of 0.24 cubic yards per two land mile." The lanes are considered to be twelve feet wide. If one fourth cubic yard of salt is estimated to weigh 450 pounds, if it is assumed that when the ground is "barely white", there is one fourth inch of snow, and if it is assumed that ten inches of snow is equivalent to one inch of water, the most concentrated brine that would be formed under the described highway practice would be a solution that contained about 2.7 percent sodium chloride. This is equivalent to 0.00355 pounds of salt per 0.1299 pounds of water per square foot of surface or 0.032 pounds of salt per square yard of surface.
Ice or packed snow becomes slick whenever a film of water forms on the surface. This film of water ordinarily forms because ice turns to water when it is put under pressure. The blade of an ice skate is a small surface area. The weight of a person presses down with a high pressure on this small surface area, causes the ice to melt, forms a thin film of water under the ice skate and allows a person to glide over the ice on a thin film of water. A several thousand pound automobile presses down on the relatively small area of four tires and slides on the water film formed on the surface of the ice.
Because of this principle, it is the practice to use granular or crystalline deicing particles to remove snow or ice from paved surfaces. If liquids were used, there is the danger of forming a film of water on the surface of the ice and of producing a hazardous slippery condition. When granular or crystalline particles of deicer are broadcast over the surface, they bore through the ice, spread a solution under the ice to loosen it from the pavement and allow the ice to be mechanically removed without having to dissolve all the ice or to leave a dangerous liquid film on the surface of the ice.
The main usage of liquid deicers is to remove the snow or ice completely from the surface. This requires many times as much deicer but it has the advantage of rapidly removing the snow or ice from the surface. Such practice is used on limited surface areas, such as to remove snow from the surface of an aircraft prior to take-off. This removes the snow without the danger of damaging the surface of the aircraft with a mechanical scraping device.
The U.S. Pat. No. 3,108,075 to Hearst, reviews the freezing points and corrosive effects of numerous organic compounds, including ammonium carbamate, and concludes that predominantly formamide mixtures are best. Hearst purported to investigate all likely materials (col. 5, lines 3-7), including ammonium carbamate, for their rates of ice melting but found, from his experiments, that "the most promising materials were formamide, ethylene glycol, and ammonium acetate." (Col. 5, lines 53-56). Hearst thus failed to recognize superior characteristics of ammonium carbamate and lead the art away from its use, apart from formamide.